Jewish and Muslim leaders here warned on Monday of rising anti-Semitism among young Muslims, two days after the police arrested 11 men and fatally shot one in raids in a handful of cities aimed at young radical French Muslims.

President François Hollande met with Jewish and Muslim leaders on Sunday and promised tighter security at Jewish religious sites. He said that a new law would soon make it a crime to travel to militant training camps. “Nothing will be tolerated; nothing should happen,” Mr. Hollande said in a statement. “Any act, any remark will be prosecuted with the greatest firmness.”